I have stored two aircraft for about 18 months and now I want to use them the engines have no compression.I assume the piston rings are stuck due to old engine oil in the cylinders. I should have used after-run oil more frequently but did not think the planes would be out of commission for so long.
One engine is an almost new OS 20 four stroke, and the other is an almost new OS 40 two stroke. How can I free these up without hopefully having to strip them down.

I would dribble a little fuel in the glow plug hole and let them sit a little, then heat them up with a heat gun and turn them over. Then I think if you start them they will free up very quickly and regain their compression.

+1-it is seldom necessary to disassemble stored engines,unless they were exposed unprotected.

Oil and heat almost always cure properly stored engines.

Most likely, Priming it with fuel, then applying the starter to it, with WOT and glow charger NOT connected, will allready free the rings after a few seconds.... (and if it doesn't, it will also not damage anything)

The quickest try, and if you have some compression after that, just set idle, connect glow and start. The rest will work itself out when the engine is running...

Brgds, Bert

It might be something as simple as a sticky valve. Pull the valve cover to see/make sure the valves are working as planned. Press the valve stem with a screwdriver, and let it snap shut a few times. Then see if the compression improves. I did this on a motorcycle that sat for a number of years. It had leaky valves. After the snap treatment it had enough compression to start and run reasonably well. After 20 minutes it was it's old self.

On that note I watched a fellow load the cylinder with oil, install the glow plug and turn the engine over BY HAND and bump a valve closed with the impending hydro lock. It was and exhaust valve and some oil squirted out before the valve shut.

You mileage may vary considerably if you try this.

Ken

Since these are 4 stroke engines, I'm more inclined to think a valve is hanging open.
Pull the valve cover and make sure both valves are closing and apply some oil to the rockers.
Pete

Also depends on type of ring. If it is a dykes type ring, they are designed to bend outward while engine is running to create the seal. Many engines with these types of rings feel as if they have no compression at all when spinning over by hand.

Originally Posted by kimchiyuk

Also depends on type of ring. If it is a dykes type ring, they are designed to bend outward while engine is running to create the seal. Many engines with these types of rings feel as if they have no compression at all when spinning over by hand.

Square rings do exactly the same: the combustion pressure gets in the ring-groove behind the ring, and pushes it outwards. The Dykes ring just builds up this outward pressure a little quicker and has to handle the total pressure difference over a single ring, but the working principle is the same: combustion pressure forcing the ring against the liner.
But square rings have a more even pressure distribution (less wear), and are less fragile.
Nowadays I have the impression, Dykes rings are not so often used anymore.

Brgds, Bert

Originally Posted by Brutus1967

Nowadays I have the impression, Dykes rings are not so often used anymore.

Brgds, Bert

It's been a VERY long time since I've seen a Dykes ringed engine. Weather it be model or otherwise.

Ken

Originally Posted by kenh3497

It's been a VERY long time since I've seen a Dykes ringed engine. Weather it be model or otherwise.

Ken

I agree, but OP didnt state age of engine

The OS fourstrokes never had Dykes rings as far as I know....

They are to my knowledge not really suitable for fourstrokes, as they tend to collapse a bit during the intake stroke.

In the '70's and '80's virtually every European model engine had them, and in small 2 stroke motorcycles, they were considered tuningparts.... I have broken a few of them Dykes rings back in the day, so I'm not really sad they ar not around so much anymore...

Brgds, Bert

Plenty of two strokes that have Dykes rings, Irvine for instance, can't speak about four strokes.
Good advices so far, check the movement of the valves, if they are fine then it is the ring and it will loosen up easily while running the engine.

The only four stroke I know of using Dykes rings was the HP VT series an I'm unsure if all of them did.

Greg

Thanks guys. Have started on the 2 stroke using fuel soak on top of piston and it seems to be a little better. Will wait and see how it goes. Have not yet started on the 4 stroke. Great input thanks.

After use, particularily before any "storage", turn the engine until you just about feel the compression.....this indicates that both valves are closed.
This way both valves will be forced to "unstick" at next start attempt...

This is a great routine with all single-cylinder 4-strokes....since I learned this trick ages ago, I have practised it with motorbikes, lawn mowers and model engines...never a stuck valve ever since...